When functioning properly, the human heart maintains its own intrinsic rhythm, and is capable of pumping adequate blood throughout the body's circulatory system. The body's autonomic nervous system regulates intrinsic electrical heart activity signals that are conducted to atrial and ventricular heart chambers on the left and right sides of the heart. The electrical heart activity signals trigger resulting heart contractions that pump blood. However, some people have irregular and uncoordinated cardiac rhythms, referred to as arrhythmias. Some of the most common arrhythmias are atrial fibrillation (AF) and atrial flutter (AFL). Atrial fibrillation can result in significant patient discomfort and even death because of a number of associated problems, including: (1) an irregular heart rate which causes the patient discomfort and anxiety, (2) loss of synchronous atrioventricular contractions which interferes with cardiac hemodynamics, resulting in varying levels of congestive heart failure, and (3) stasis of blood flow, which increases the vulnerability to thromboembolism.
One mode of treating cardiac arrhythmias uses drug therapy. Drugs are often effective at restoring normal heart rhythms. However, drug therapy is not always effective for treating arrhythmias of certain patients. For such patients, an alternative mode of treatment is needed. One such alternative mode of treatment includes the use of a cardiac rhythm management system. Such a system may be implanted in a patient to deliver therapy to the heart.
Cardiac rhythm management systems include, among other things, implanted rhythm management devices. Implanted rhythm management devices deliver, among other things, timed sequences of low-energy electrical stimuli, called pace pulses, to the heart, such as via a transvenous lead wire or catheter (referred to as a “lead”) having one or more electrodes disposed in or about the heart. Coordinated heart contractions can be initiated in response to such pace pulses (this is referred to as “capturing” the paced heart). By properly timing the delivery of pace pulses, the heart can be induced to contract in a coordinated rhythm, greatly improving its efficiency as a pump. Such devices are often used to treat patient's hearts exhibiting arrhythmias. Implanted rhythm management devices are also used to deliver high-energy defibrillation pulses via a lead wire having one or more electrodes disposed in or about the heart for providing defibrillation therapy.
Implanted rhythm management devices generally include sensing circuits to sense electrical signals from a heart tissue in contact with the electrodes. Then a controller in the implanted rhythm management device processes these signals and issues command signals to therapy circuits, for delivery of electrical energy such as pacing and/or defibrillation pulses to the appropriate electrodes in or about the heart to provide therapy to the heart. The controller may include a microprocessor or other controller for execution of software and/or firmware instructions. The software of the controller may be modified to provide different parameters, modes, and/or functions for the implantable device to adapt or improve performance of the device. Generally algorithms are used in software and/or firmware residing in the controller to discriminate between sensed coordinated and uncoordinated cardiac signals and to provide an appropriate therapy to the heart. Current techniques to discriminate cardiac rhythms in the sensed cardiac signals are based on interval information and ignore serial interval relationships in the sensed cardiac signals. Thus, a need exists for a more reliable, more sensitive method of discriminating cardiac rhythms in the sensed cardiac signals in implanted rhythm management devices to provide the appropriate therapy (whether to deliver pacing pulses or high-energy therapy) to the heart and to reduce patient morbidity and discomfort. Also, what is needed is an implanted rhythm management device that can save electrical energy and reduce patient discomfort by delivering high-energy defibrillation pulses only when lower energy therapies such as anti tachycardia pacing (low energy pacing) are not likely to restore normal function to the heart.